At a Westminster press conference, they made clear that their concerns about May’s performance went far wider, accusing the PM of throwing away the modernisation agenda begun by David Cameron and allowing the party to be taken over by right-wing Brexit hardliners.

Soubry said she would not stay in the Conservatives to “skirmish on the margins when the truth is the battle is over and the other side has won”.

She said: “The right wing, the hardline anti-EU awkward squad that have destroyed every leader for the last 40 years are now running the Conservative Party from top to toe. They are the Conservative Party.”

The Broxtowe MP urged “fellow one-nation Conservatives” and “like-minded Lib Dems” to “please, come and join us” by breaking away from their parties and joining the new grouping.

She accused a “purple Momentum” of hard-right “zealots” of trying to force out MPs on the Remain wing of the party through deselections.

Speaking ahead of their resignations, Tory grandee Kenneth Clarke said that he could think of “half a dozen” Conservatives who might quit the party unless it changed direction. May could face “quite a lot” of ministerial resignations unless she ruled out a no-deal Brexit, he said.

The Father of the Commons said the prime minister was “running the risk of losing moderates from her party” and was “simply acting at the behest of the extreme right wing of the party”.

Allen described the Tory trio as the “three amigos” who had joined the “magnificent seven” ex-Labour MPs who launched The Independent Group (TIG) on Monday and the “lone ranger” Joan Ryan who joined them last night.

She said she believed “a significant number” of Conservative MPs were considering joining them.

Soubry urged ministers to quit in order to vote against a no-deal Brexit.

She said: “I’m hoping that this will really concentrate some minds of colleagues in the Conservative Party that we know share our concerns and also share our values and our principles and are very unhappy about the direction of travel.

“I also hope it gives courage to members of the government who are deeply concerned about this no-deal becoming a real possibility.

“And it will give them the courage next week to do what, frankly, some of them should have done a long time ago and be true to what they believe, and if they need to leave government and vote against the party line on Brexit, they have got to do it.”

The move brings the tally of TIG MPs to 11 - equal to the Liberal Democrats and one more than the Democratic Unionist Party, who prop up May’s minority administration in the House of Commons.

In a letter to May, the three MPs said: “We no longer feel we can remain in the party of a government whose policies and priorities are so firmly in the grip of the ERG [European Research Group] and DUP.

“Brexit has re-defined the Conservative Party - undoing all the efforts to modernise it. There has been a dismal failure to stand up to the hard line ERG which operates openly as a party within a party, with its own leader, whip and policy.”

They added: “The final straw for us has been this government’s disastrous handling of Brexit.”

Former minister Soubry, the MP for Broxtowe, Totnes MP Wollaston and South Cambridgeshire MP Allen said they could “no longer act as bystanders” as May continued with her Brexit strategy.

They said: “Following the EU referendum of 2016, no genuine effort was made to build a cross-party, let alone a national consensus to deliver Brexit.

“Instead of seeking to heal the divisions or to tackle the underlying causes of Brexit, the priority was to draw up ‘red lines’. The 48% were not only sidelined, they were alienated.”

They added that there was a wider dissatisfaction with the state of British politics, with both the Tories and Labour moving “to the fringes, leaving millions of people with no representation”.

They said “there will be times” when they would support the government at Westminster “on measures to strengthen our economy, security and improve our public services”.

“But we now feel honour-bound to put our constituents’ and country’s interests first.”

Speaking at the press conference, Allen, a member of the Work and Pensions Select Committee, made clear she was also opposed to the government’s stance on welfare.

“I can no longer represent a government and a party who can’t open their eyes to the suffering endured by the most vulnerable in society, suffering which we have deepened whilst having the power to fix,” she said.

Wollaston, who chairs the Commons Health Committee, said: “I am afraid the prime minister simply hasn’t delivered on the pledge she made on the steps of Downing Street to tackle the burning injustices in our society.

“I think that what we now see is the party, that was once the most trusted on the economy and business, is now marching us to the cliff-edge of a no-deal Brexit.”

May said she was “saddened” by the decision but insisted that “we are doing the right thing for our country” by delivering Brexit.

She added: “I am determined that under my leadership the Conservative Party will always offer the decent, moderate and patriotic politics that the people of this country deserve.”

A Downing Street spokesman said the three MPs’ Conservative associations were now free to select new general election candidates but said he was not aware of any central directive to do so. He said he was not in a position to rule out their return to the party after Brexit.

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